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Fair value measurements (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018	Sep. 30, 2018
Fair value measurements
Marketable securities	$ 53,806	$ 10,497
Transfers between fair value measurement level 1 to level 2	0		$ 0
Transfers between fair value measurement level 2 to level 1	0		$ 0
Recurring basis
Fair value measurements
Total assets regularly measured and carried at fair value	53,806	10,497
Recurring basis \| Commercial paper
Fair value measurements
Marketable securities	14,871
Recurring basis \| Corporate bonds
Fair value measurements
Marketable securities	12,010
Recurring basis \| U.S. Treasuries
Fair value measurements
Marketable securities	26,925	10,497
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1)
Fair value measurements
Total assets regularly measured and carried at fair value		10,497
Recurring basis \| Quoted Prices in Active Markets for Identical Assets (Level 1) \| U.S. Treasuries
Fair value measurements
Marketable securities		$ 10,497
Recurring basis \| Significant Other Observable Inputs (Level 2)
Fair value measurements
Total assets regularly measured and carried at fair value	53,806
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Commercial paper
Fair value measurements
Marketable securities	14,871
Recurring basis \| Significant Other Observable Inputs (Level 2) \| Corporate bonds
Fair value measurements
Marketable securities	12,010
Recurring basis \| Significant Other Observable Inputs (Level 2) \| U.S. Treasuries
Fair value measurements
Marketable securities	$ 26,925